Materials with controlled size pore structures on the atomic dimension have been used as molecular sieves for sorption, catalysts, and ion exchange resins. The most well known of these materials are zeolites which is a name derived from the Greek, meaning boiling stones. The controlled pore structure of zeolites result from the chemical arrangement of (Al,Si)O.sub.4 tetrahedra which share all their oxygen vertices with nearby tetrahedra and are joined together to give rise to large cavities and controlled size windows into these cavities. The alumino-silicate framework forming the zolite usually has a negative charge which is balanced by alkalies and alkaline earths located outside the tetrahedra in the channels. These materials have proven to be useful in a variety of industrial applications because of the shape and chemistry of the pore structures formed by the linked alumino silicate tetrahedra. Pores in these materials can be monodisperse and small enough to act as molecular sieves so that different apparent surface areas are obtained according to the size of the absorbate molecules. The chemistry of the framework and the counter ions neutralizing electrical charge on the framework can have many catalytic applications. In particular, the controlled pore structure can produce shape selective effects in catalysis (see, e.g., N.Y. Chen, U.S. Pat. No. 3,630,966, Dec., 28, 1971).
In all zeolites, the chemical composition is intimately related to the size of the pore structure. Maximum pore size in zeolites is related to the geometric arrangement of the alumino-silicate tetrahedra and is always less than about 10 .ANG..
The present invention describes zeolite like materials made using physical fabrication techniques. Physical fabrication techniques such as etching, deposition and lithographic patterning have been extensively used for the production of microelectronics. Features with approximately one micron critical dimensions are routinely created using these methods for microelectronic circuitry; however, to produce porous materials capable of molecular sieving requires reducing the feature size by three orders of magnitude.
Reduction in feature size is obtained by using new methods for defining the pattern used with thin film etching and deposition techniques. By using physical fabrication techniques to produce controlled porosity on the molecular dimension, new degrees of freedom in constructing zeolite like materials are obtained. Physical fabrication techniques decouple the interrelationship between size and chemistry of the pore structures. Thus, the composition of zeolite like materials made with physical fabrication techniques is not limited to aluminosilicates. Using the physical fabrication techniques described herein it is possible to make zeolite-like materials from a wide variety of semiconductors, metals and insulators. Shape of pore structures made with physical fabrication techniques can be significantly different from those in natural and synthetic aluminosilicate zeolites. For example, the pore structure occuring in the physically fabricated etched superlattice structure described herewith is two dimensional rather than one dimensional as is the case for conventional zeolite materials. Precise control of pore size can be obtained using physical fabrication allowing for a choice of critical dimensions in the size range from approximately 10 to more than 10,000 .ANG.. This is a size range not readily accessible with conventional zeolite materials. With these broad ranges of flexibility of construction, etched superlattice zeolite-like materials made by physical fabrication techniques have a control over pore size and chemistry which is not available with conventional aluminosilicate zeolites. Since pore size and chemistry are the determining factors in use of zeolites for separations and catalysis, etched superlattice zeolite-like materials will have many inherent advantages in these areas. In particular, etched superlattices provide a new class of micro-porous shape selective materials that significantly expand the range of behavior spanned by conventional zeolites. The term "shape selective" or "shape selective activity" is taken throughout this patent to mean a material whose interaction with chemical molecules can be different depending on the molecular size or shape.